Preparation of Solvent-free Polyurethane Adhesive
One-component solvent-free polyurethane adhesive, as usual, does not need to be prepared and can be used directly according to operational requirements, which is also the reason why one-component adhesive is easy to use. For two-component or multi-component solvent-free polyurethane adhesive, it should be prepared according to the instructions. If the hydroxyl group content and the isocyanate group content are known, the distribution ratio of each group can be determined by stoichiometric calculation. The isocyanate index R = NCO/OH is generally in the range from 0.5 to 1.4.
single componentSolvent-free polyurethane adhesiveAs usual, there is no need to prepare, and it can be used directly according to the operation requirements, which is also the reason why the one-component adhesive is easy to use. For two-component or multi-component solvent-free polyurethane adhesive, it should be prepared according to the instructions. If the hydroxyl group content and the isocyanate group content are known, the distribution ratio of each group can be determined by stoichiometric calculation. The isocyanate index R = NCO/OH is generally in the range from 0.5 to 1.4.
Generally speaking, when preparing two-component solvent-based polyurethane adhesive, the ratio tolerance of the two components is larger than that of non-solvent type. However, if there are too many NCO groups in the adhesive, the curing is incomplete, and the cured adhesive layer is hard or even brittle. If there are too many hydroxyl components, the adhesive layer is soft and sticky, the bonding force is low, and the bonding strength is poor. The tolerance of solvent-free two-component adhesive ratio is smaller than that of solvent-based two-component adhesive ratio, which is due to the small initial molecular weight of each component. If a component is excessive, the curing is slow, it is not easy to complete, and the adhesive layer has low adhesive strength.
Due to the limited service life of the glue, the prepared glue takes several days to run out. The coating period is the time during which the adhesive retains its operable structure after preparation. The viscosity increases with time, making it difficult to handle until the gel loses fluidity and the gel fails. Different varieties and brands of solvent-free polyurethane adhesives have different application cycles, ranging from a few minutes to a few days. When used in large quantities in industrial production, the pot life test should be carried out in advance.
If the adhesive component contains a catalyst, or in order to speed up the curing speed of the catalyst added to the glue, the application cycle is shorter. In addition, the environmental temperature has a great influence on the application period, with short summer and long winter. Two-component solvent-free polyurethane adhesive is diluted with urethane grade organic solvent. Such as commonly used solvent-based two-component adhesive, such as soft plastic composite film with two-component solvent-free polyurethane adhesive, the application cycle should be greater than 8h (I. e. a working day).
If the good glue is not used up on the same day, it can be diluted appropriately, covered and closed, stored in a cool place, and checked for turbidity or signs of gel when going to work the next day. If there is no obvious change on the surface of the glue and the fluidity is good, it can still be used, and a small amount of people can be mixed into the new glue as usual. If it has deteriorated, it should be discarded.
In order to reduce the viscosity, convenient operation, make the coating uniform, and help control the sizing thickness, organic solvent can be added for dilution. Solvent-free polyurethane glue can be used as diluents, such as methyl ethyl ketone, toluene, ethyl acetate, etc.
Adding a catalyst can accelerate the curing speed of the adhesive. The curing catalyst is typically an organotin compound.
For solvent-based solvent-free polyurethane adhesive, the film needs to be dried for a few minutes to tens of minutes, so that most of the solvent in the adhesive volatilization, which is conducive to improving the initial adhesion. If necessary, it should be properly heated and air-dried (such as composite film coating process). Otherwise, due to a large amount of solvent remaining in the adhesive, it is easy to form bubbles in the adhesive layer during the curing process, which affects the bonding quality. Solvent-free polyurethane adhesive, can be bonded after bonding.
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